Modulated-carrier signal receiver



H. MQLEWIS MODULATED-CARRIER SIGNAL RECEIVER ,Aug. 19, 1941.

Filed Sept. 25, 1939 Ow mn ATTORNEY Patented `A'ugrl9, 19141 I UNITED STATES PrimeurI orifice MDULATED-CABBIEB SIGNALBECEEB Hmmm. ma, cire-u Nesi, N. y., una u Hueltine Corporation, a corporationy of Dela- Application September 25, 1939, Sei-iai No. 296,339

12 Claims.

I ticular utility in a modulated-carrier signal receiver adapted for the reception of a frequencymodulated signal which is also subject toundesired amplitude modulation.

Receivers for the reception of a frequency-` modulated wave generally incorporate therein an arrangement for substantially eliminating amplitude modulation of the received signal and for thereafter applying the signal to an analyzer circuit wherein the frequency-modulation components are detected and converted into amplitude modulation for detection by means of a conventional amplitude detector. In receivers of this type, it is, of course, essential to eliminate all incidental and undesired amplitude modulation. Lirniters have commonly been utilized for this purpose at various points in the signal-translating 1channel of the receiver. However, such limiters do not provide an entirely satisfactory operation for the reason that the circuits of the limiters generally -comprise vacuum tubes operated beyond their upper or lower cutoi points to eliminate amplitude modulation of the translated signal and, due to the curved input voltageoutput current characteristic of such limiter tubes in the region of either cuto point, an appreciable distortion is eiected within the tube which, in turn, may be reproduced as an undesired signal disturbance.

It is an object of the present invention, therefore, to provide an improved modulated-carrier signal receiver adapted to receive a carrier having a first characteristic type of modulation representing a desired signal and subject to undesiredmodulation of a second' characteristic type.

It is a further object of the invention to provide an improved modulated-carrier signal receiver adapted to receive a frequency-modulated signal and including means for substantially eliminating the effect of undesired amplitude modulation.- I

In accordance with the invention, therefore, a modulated-carrier signal receiver includes means for selecting a carrier wave having a rst (Cl. Z50-20) characteristic type of modulation representing a desired signal and subject to undesired modulation of a second characteristic type, a carrier frequency signaling-translating stage, and means for selectively detecting and utilizing the selected carrier modulation ofthe first type to` re-v produce the desired'signal.` There is further provided means for selectively detecting and utilizing only `the selected carrier modulation of the second type instantaneously to control the response characteristic of the carrier-frequency signal-translating stage inversely in accordance therewith to reduce \the effects of the undesired signal modulation. f In a preferred embodiment of the invention, the detected components of the selected carrier modulation of the second type are utilized instantaneously and inversely to controlthe gain of one or more amplifier tubes in the carrier-frequency signal-translating channel in the receiver to reduce the eects of the undesired signal modulation.

For a better understanding of the present invention, together with other and further objects thereof, reference is had .to the followingdescription taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Fig. 1 of the drawing is a circuit diagram,

partly schematic, of a complete superheterodyne receiver adapted to receive a frequency-modulated carrierwave and comprising therein an arrangement `in accordance with the invention for suppressing undesired amplitude-modulation components of the received signal, while each of Figs. 2 and 3 illustrates a different embodiment of the invention.

Referring now' more particularly to Fig. 1 of the drawing. there is shown a circuit diagram.

partly schematic, of a superheterodyne receiver embodying the invention and adapted to receive a carrier'wave having a ilrst characteristic type of modulation representing a desired signal and subject to undesired modulation of a second characteristic type. Specifically, the receiver is adapted to receive a frequency-modulated carrier wave which is subject to undesired amplitude modulation. Briefly described, the receiver comprises a radio-frequency amplifier I0 having input terminals connected to a doublet antenna II, I2 and output terminals coupled to a modulator or frequency-changer unit I3 having associated therewith a local oscillator I4. Connected in cascade with the output circuit of modulator I3, in the order named, are an intermediate-frequency amplier I5 of 'one or more stages, a second interquency changer I3. The signal as thus converted is further amplified in intermediate-frequency amplifiers I5, I3, and I1 and detected by 'the frequency-modulation detector I3, thereby producing the audio-frequency modulation components which are, in turn, amplified by audio-frequency amplifier I9 and reproduced by the sound reproducer 20. Thus, the frequency-modulation detector I3, the audio-frequency amplifier I9, and the sound reproducer comprise means for selectively detecting and utilizing the selectedcarrier modulation of the type representing the desired signal to reproduce the latter.

Referring now more particularly to the details of the system comprising the present invention, the stage of intermediate-frequency amplication I5 includes therein a vacuum tube 25 having fnput terminals coupled through transformer 26 to the output terminals of intermediate-frequency amplifier I5 and output terminals coupled to the input terminals of intermediate-frequency amplifier I1. A suitable self-bias arrangement, in-

cluding cathode-resistor 21, by-passed for highfrequency signals by condenser 28, is provided for vacuum tube 25. In order to derive the amplitude-modulation components ofthe received signal, there is provided means coupled tothe signal-translating channel I5 for selectively detecting the undesired amplitude-modulation components of the received carrier signal comprising a diode detector 29 coupled to the output circuit of intermediate-frequency amplifier I 5 vby means of a transformer 30. Detector 29 comprises e load circuit including delay-bias source 29 and resistor 3|, by-passed for carrier-frequency signals by condenser 32. The amplitude-modulation components of the signal are thus derived from load resistor 3I and are applied positively to a direct coupled vacuum-tube amplifier 33 through an inductance 34. A suitable biasing arrangement comprising cathode-resistor 35, by-passed for carrier-frequency signals by condenser 35, is provided for vacuum tube 33 while the control-grid circuit includes a resistor 31, by-passed by condenser 33. It follows that elements 3|, 32, 34, 31, 38 effectively comprise a properly terminated low-pass filter circuit to develop the signal voltage across the input circuit of tube 33. Suitable operating potentials are provided for tubes and 33 in a manner well understood in the art.

The signal output of tube 33 is utilized to control the gain of the signal-translating channel of the receiver inversely in accordance withthe amplitude-modulation components of the received signal, thereby to cause an amplitude demodulation of lthe received signal and effectively reduce its resultant amplitude modulation to an vunobiectionable value. For this purpose, the signal output of tube 33 is applied tothe screen grid 40 of tube 25. The output voltage of tube 33 furthermore may be applied ln a similar manner to one or more of the tubes in radio-frequency amplifier I0, modulator I3, and intermediatefrequency amplifiers I5 and I1, and is illustrated as being applied to all of these tubes by way of a conductor 4I.

Oscillator I4 comprises a vacuum tube 42 having a frequency-determining circuit 43, 44 and an oscillation-sustaining feed-back circuit including a condenser 45 and an inductance 46 inductively coupled to inductance 43. 'I'he output circuit of the oscillator I4 is coupled to modulator I3 through an inductance 41 inductively coupled to inductance 43. The amplitude-modulation components of thereceived signal derived from the output circuit of vacuum tube 33 are shown as applied also to the screen-grid electrode 49 of vacuum-tube oscillator 42. The frequency-modulation translating channel is preferably designed for a wide band frequency-modulation receiver to pass a wide frequency range, while the circuits including tubes 29 and 33 are designed to pass a relatively narrow band, preferably of the same order of magnitude as the band width of the desired audio-frequency-signals, since it may not be necessary to balance out amplitude-modulation components outside the audio-frequency range.

In considering the operation of the portion of the system of Fig. 1 comprising the present invention, it will be seen that the amplitude-modulation components of the received signal are derived from the load circuit of detector 29 and amplified by vacuum tube 33 to provide a signal output therefrom which varies in accordance with the instantaneous values of the amplitudemodulation components of the received signal. These amplitude-modulation components are applied to the screen grid of vacuum tube 25 in such a way as to vary the gain thereof inversely in accordance with the amplitude of the amplitudemodulation components ofthe received signal, thereby to demodulate or reduce the percentage of amplitude modulation of the received signal. The output signal of vacuum tube 25, as applied to intermediate-frequency amplifier I1, therefore, has the amplitude-modulation components thereof materially reduced.

The tubes of radio-frequency amplifier III, modulator I3, and intermediate-frequency amplifiers I5 and I1 may be controlled in a similar manner, thereby to eliminate substantially all amplitude-modulation components of the received signal as applied to the frequency-modulation de- .tector I9, thereby eliminating the necessity for limiter arrangements in the signal-translating channel oi' the receiver comprising vacuum tubes operated through their cutoff points with their attendant disadvantages. Also, the output of vacuum tube 33 is applied to the screen electrode of oscillator tube 42 to vary the amplitude of the output thereof inversely in accordance with the amplitude of the amplitude-modulation components of the received signal. 'I'his also has the effect of decreasing the amplitude-modulation components of the received signal as applied to the intermediate-frequency channel of the receiver. It will be seen that the control of the gain of the signal-translating stages I9, I 3. and I5 occurs at points preceding the point to which the detector 29 is coupled to the signal-translating channel and. therefore, is regressive as applied to these units. On the other hand, the control of the gain of tube 25 and intermediatefrequency amplifier I1 occurs at points succeeding the point at which the detector 2l iscoupled to the signal-translating channel and is progressive as applied to these signal-translating stages, which condition is necessary for a Vsubstantially complete elimination of amplitude modulation of the received signal. Thus, the detector 2! and the control connections to the carrier-frequency signal-translating stages I0, Il, and l5|1, inclusive, comprise means for selectively detecting and utilizing only the selected-carrier modulation of the type representing undesired amplitude modulation to control or reduce the amplitude response or gain characteristic of the carrier-frequency signal-translating stages lll. I3, and I5-I1, inclusive, in accordance with instantaneous values thereof to reduce the effects of the `undesired amplitude modulation of the received signal effectively by amplitude demodula- -tion of the signal.

It will -be seen that, inasmuch as the control circuit including tubes `29 and 33 is effective at very low frequencies, the system of Fig. 1 is effective also to provide an automatic volume control for the receiver, and, due to the bias provided by source 29', the amplitude of the inputv to detector i8 is maintained within a relatively narrow range for a wide range of input amplitudes of the receiver, which may vary at a very low frequency.

The embodiment of the invention illustrated in Fig. 2 is generally similar to that illustrated in Fig. 1 and similar circuit elements have identical reference numerals. The embodiment of the in-y vention illustrated in Fig. 2 dilers from that of Fig. 1 primarily in that the amplitude-modulation components of the received signal are applied to the suppressor electrode 51 of tube 25 rather than to the screen-grid electrode 4I as in Fig. 1. Inasmuch as amplitude-modulation components of the same polarity are necessary for a like effect upon the gain -of\vacuum tube 25 whenapplied to the suppressor electrode 51, the

stage of intermediate-frequency amplification I64 -of the circuit of Fig. 2 unnecessary. Inasmuch as it is desirable that the amplitude-modulation components applied to grid 51 of tube 25 be substantially in phase with corresponding components applied through transformer 26 to the input electrode of tube 25, it may become necessary to provide some arrangement for adjusting the relative phase of these signals. Such compensating network or phase changer 6l is shown in the circuit of Fig. 2 coupled between inductance 60 and grid 51. Phase changer Glv may be of conventional design.

In Fig. 3 there is illustrated a further rndica-l tion of the invention `wherein elements which are similar to the elements of the preceding figures have identical referencenumerals. The circuit of Fig. 3 diiiers from that of Fig. 2 mainly in that amplifier 53 and phase changer 6I are omitted and in that the output potentials of a polarity opposite to that of corresponding amplitude-modulation components applied to the input circuit of tube 25 directly throughtransformer 26. The operation of the circuit of Fig.` 3 is otherwise similar to that of Fig. 2. rehder` ing a detailed description of the operation thereof unnecessary.

While there have been described what are at present considered to be the preferred embodiments of this invention, it willbe obvious to those skilled in the art that various changes and modincations may be made therein without departing from the invention, and it is. therefore, aimed in the appended claims to cover all such changes `and modifications as fall within the true spirit and scope of the invention.`

What is claimed is:

`1. A modulation-carrier signal receiver adapted to receive a carrier wave having a rst characteristic type of modulation representing a desired signal and subject to undesired modulation of a second characteristic type comprising, a carrier-frequency signal-translating stage, means for selectively detecting and utilizing said selected-carrier modulation'of said first type to reproduce the desired signal, and means for selectively detecting and utilizing only said selectedcarrier modulation of the second type to control the response characteristic of said carrier-frequency signal-translating stage in accordance with instantaneous values thereof to reduce the effects of said undesired signal modulation.

2. A modulated-carrier signal receiver having a signal-translating channel including a carrierfrequency stage and adapted to 'receive a carrier wave havingy a rst ,characteristic type of modulation representing a desired signal and subject to undesired modulation of a second characteristic type comprising, means for selectively detecting and utilizing said selected-carrier modulation of said first type to reproduce the desired with.

signal, and means for selectively detecting and utilizing only said selected-carrier modulation of said second type to control the gain of said carrier-frequency stage inversely in accordance with the instantaneous values thereof to reduce the effects of said undesired signal modulation.

3. A modulated-carrier signal receiver having a signal-translating channel including a carrierfrequency stage and adapted to receive a carrier wave having a rst characteristic type of'modulation l.representing a desired signal and subject to undesired modulation of a second characteristic type comprising, means for selectively detecting and utilizing said selected-carrier modulation of said first type to reproduce the desired signal, and means for selectively detecting and utilizing only said selected-carrier modulation of said second type instantaneously@ control said carrier-frequency stage in accordance with the instantaneous values thereof to provide demodulation ofthe received signal in accordance there- 4. A modulated-carrier signal receiver including a signal-translating channel including a carrier-frequency stage and adapted tol receive a carrier wave having a first characteristic type of modulation representing a desired signal and subject to undesired modulation of a second characteristic type comprising, means for selectively detecting and utilizing said selected-carrier modulation of the first type to reproduce the desired signal, means coupled to said signaltranslating channel for selectively detecting the modulation components of said selected-carrier modulation of the second type, and means for utilizing said detected modulation components to reduce the gain of said carrier-frequency stage in accordance with the instantaneous values thereof at a point succeeding the point to which said last-mentioned detecting means is coupled, thereby to reduce the eiects of said undesired signal modulation.

5. A modulated-carrier signal receiver including a signal-translating channel including a carrier-frequency stage adaptedto receive a carrier wave having a first characteristic type of modulation representing a desired signal and subject to undesired modulation of a second characteristic type comprising, means for selectively detecting and utilizing said selected-carrier modulation of the first .type to reproduce said desired signal, means coupled to 'said signal-translating channel for selectively detecting the modulation components of said selected-carrier modulation of said second type, and means for utilizing said detected modulation components to reduce the gain of said carrier-frequency stage in accordance with the instantaneous values thereof at a point preceding the point to which said lastmentioned detecting means is coupled, thereby to reduce the eiects of said undesired signal modulation.

ing a signal-translating channel including carrier-frequency stages adapted to receive a carrier Wave having a first characteristic type of modulation representing a desired\ signal and subject to undesired modulation of a second characteristic type comprising, means for selectively detecting and utilizing said selected carrier modulation of the first type to reproduce said desired signal, means coupled to said signaltranslating channel for detecting the modulation componentsl of said selected-carrier modulation of said second type, and means for utilizing said detected modulation components to reduce the gain of said carrier-frequency stages in accordance with the values thereof at a point preceding and at a point succeeding the point to which said last-mentioned detecting means is coupled.

thereby to reduce the effects of said undesired signal modulation.

I 7. A modulated-carrier signal receiver of the superheterodyne type including a local oscillator adapted to receive a carrier wave having a first characteristic type of modulation representing a desired signal and subject to undesired modulation of a second characteristic type comprising, means for selectively detecting and utilizing said selected carrier'modulation of said first 'type to reproduce the desired signal, means for selectively detecting said selected-carrier modulation of said second type, and means for utilizing the detected modulation components of said second type to vary the amplitude of the output of said oscillator in accordance with the instantaneous values thereof and thereby to reduce the effects of said undesired signal modulation.

' 8. A modulated-carrier signal receiver includving a signal-translating channel adapted to receive a carrier wave having a first characteristic 75 type of modulation representing a desired signal and subject to undesired modulation of a second characteristic type comprising, carrier-frequency amplifier means in said channel including a vac- 9. A modulated-carrier signal receiver adapted n to receive a carrier wave having a frequency modulation representing a desired signal and subject to undesired amplitude modulation comprising, a carrier-frequency signal-translating stage, means for selectively detecting and utilizing .said selected carrier frequency modulatibn to reproduce the desired signal, and means for selectively detecting and utilizing only said selected-carrier amplitude modulation to control the response characteristic of said carrier-frequency signal-translating stage inversely in accordance with the instantaneous values thereof. whereby the effects of said undesired signal modulation are materially reduced.

10. A modulated-carrier signal receiver adapted to receive a carrier Wave having a frequency modulation representing a desired 'signal and subject to undesired amplitude modulation comprising, a carrier-frequency signal-translating stage, means for selectively detecting and utilizing said selected-carrier frequency modulation to reproduce the desired signal, and means for Selectively detecting and utilizing said selectedcarrier amplitude modulation to control the response characteristic of said carrier-frequency signal-translating stage inversely in accordance with the instantaneous values thereof and for varying said characteristic inversely in accordance with the amplitude of the signal input to said receiver, whereby the effects of said undesired signal modulation are materially reduced and an automatic gain control is provided for said receiver.

11. A modulated-carrier sig-nal receiver adapted to receive a carrier wave having a frequency modulation representing a desired signal and subject to undesired amplitude modulation comprislng, a carrier-frequency signal-translating stage, means for selectively detecting and utilizing said selected-carrier frequency modulation to reproduce the desired signal, and means including a delay-bias source for selectively detecting and utilizing only said selected-carrier amplitude modulation to control a response characteristic of said carrier-frequency signal-translating stage inversely in accordance with the instantaneous values thereof and for varying said characteristic inversely in accordance with the amplitude of the signal input to said receiver, whereby the eect of said undesired signal modulation is materially reduced and an automatic gain control is provided for said receiver.

12. A wide band frequency-modulation signal receiver adapted to receive a carrier wave having a frequency modulation corresponding to a wide frequency range representing a desired signaland subject to undesired amplitude modulation comprising, a carrier-frequency signaltranslating stage, means for selectively detectthe response characteristic of `said carrier-frequency signal-translating stage inversely in accordance with the instantaneous values thereof. whereby the effects of said undesired signal mod- 5 ulaton are materially reduced.

HAROLD M. LEWIS. 

